1. Field
The present disclosure relates generally to coaxial tubing used in vehicles, such as aircraft. More particularly, the present disclosure relates to forming an electrostatic connection between the outer tube and the inner tube of coaxial tubing without creating an inconsistency in the tubing surfaces.
2. Background
Tubing is used to carry various fluids through aircraft and other vehicles. For example, tubing may be used in an aircraft to carry hydraulic fluid that is used by hydraulic systems on the aircraft. As another example, tubing is used in an aircraft to carry fuel between fuel tanks and the aircraft engines. Tubing may be used to carry other fluids on aircraft and other vehicles.
It is desired that the tubing used on aircraft may have various characteristics that improve aircraft performance and safety. For example, using lighter tubing on an aircraft may improve aircraft performance by improving fuel efficiency. Using tubing that is more resistant to undesirable inconsistencies may improve aircraft performance by reducing the need for reworking or replacing tubing with undesirable inconsistencies. Using tubing that is more resistant to undesirable inconsistencies also may improve aircraft safety.
Tubing used on aircraft may be made of materials that provide the desirable characteristics of lighter weight and improved resistance to undesirable inconsistencies. For example, tubing made of titanium provides these desired characteristics. Tubing made of other materials or of various materials in combination also may provide these characteristics.
It also is desirable that tubing on aircraft provides thermal insulation between the fluid carried in the tubing and adjacent structures of the aircraft through which the tubing is run. Such insulation is desirable to reduce heat transfer between the fluid carried in the tubing and the adjacent aircraft structures. Heat transfer between the fluid carried in the tubing and adjacent aircraft structures may affect the fluid, the adjacent structures, or both in undesired ways.
Thermal insulation between the fluid carried in tubing and adjacent aircraft structures may be provided by the use of coaxial tubing. Coaxial tubing includes an inner tube that is surrounded by an outer tube. A fluid, such as hydraulic fluid, fuel, or another fluid, is carried in the inner tube. The outer tube is separated from the inner tube to form a channel between the inner tube and the outer tube. This channel forms a gap between the tubes that may be filled with a thermally insulating material. For example, the channel between the tubes may be filled with air, another thermally insulating gas, with another thermally insulating fluid, or with other materials.
The use of coaxial tubing on aircraft also may improve aircraft safety. For example, an undesired leak of fluid from the inner tube of the coaxial tube will flow or otherwise be discharged into the channel between the inner tube and the outer tube. The leak of fluid from the inner tube is contained by the outer tube of the coaxial tube in the channel between the tubes. The outer tube of the coaxial tube thus prevents the leak of fluid from the inner tube from reaching other aircraft components, which otherwise might affect operation of the other aircraft components in undesired ways.
The air or other thermally insulating material in the channel between the tubes of a coaxial tube used in an aircraft typically also is electrically insulating. The tubes of the coaxial tube used in the aircraft may be made of an electrically conducting material, such as titanium or another electrically conducting material. In this case, in some operating environments or conditions, an electrical charge or current may form on one or both of the tubes of the coaxial tube. For example, a lightning strike on the aircraft may cause such a build-up of charge or current in the coaxial tube. Since the tubes in the coaxial tube are separated by an electrically insulating channel and charge cannot flow freely between the tubes, this build-up of charge or current may cause an electrical discharge in the form of a spark across the channel between the tubes. Such a discharge is undesirable. For example, any fuel vapor or other combustible material in the channel between the tubes of the coaxial tube may be ignited by a spark across the channel.
Current methods and systems for preventing an electrical discharge across the channel between the tubes of a coaxial tube may not remain fully effective in various conditions or over extended periods of time. Furthermore, current methods and systems for preventing an electrical discharge in a coaxial tube may affect the performance of the tube in undesired ways.
Accordingly, it would be advantageous to have a method and apparatus that takes into account one or more of the issues discussed above, as well as other possible issues.